In a conventional fuel injection valve, a coil is energized to generate magnetic force, so that a fixed core attracts a movable core. The movable core reciprocates with a valve member, so that the fuel injection valve intermittently injects fuel through a nozzle hole.
The fuel injection valve needs to accurately control an amount of fuel by enhancing responses in opening and closing the valve member even when the fuel injection valve injects a small amount of fuel.
For example, a fuel injection valve disclosed in U.S. Pat. No. 6,712,297B1 (JP-A-2002-48031) has a structure, which is similar to a structure of a fuel injection valve 200 shown in FIGS. 10A, 10B. The fuel injection valve 200 includes a coil 220 having a winding 222, which is substantially circular in cross section. When the circular cross sectional winding 222 is wound to construct the coil 220, wires of the winding 222, which are adjacent to each other, form a gap 230 therebetween. As a result, a space factor of the winding 222 decreases. Here, the space factor represents a rate of a space occupied by the winding with respect to a space in the coil. Therefore, when the space factor is low, the gaps formed in the winding 222 become large. When the inner diameter of the coil 220 is the same, and when the space factor decreases, one of the outer diameter and the axial length of the coil 220 increases. As a result, a magnetic path, through which magnetic flux generated using the coil 220, becomes large. In this case, the magnetic flux is apt to leak, and magnetic force may decrease. Accordingly, the movable core 210 and the valve member 202 may not be quickly attracted toward the fixed core 212. Thus, it is difficult to enhance the responses in opening and closing the valve member 202. In addition, the fuel injection valve 200 is hard to be small sized when the outer diameter and the axial length of the coil 220 increases.